South Texas Coastal Area Storm Surge Model Development and Improvement
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AIMS Geosciences, 6(3): 271–290. DOI: 10.3934/geosci.2020016 Received: 01 June 2020 Accepted: 17 July 2020 Published: 21 July 2020 http://www.aimspress.com/journal/geosciences Research article South Texas coastal area storm surge model development and improvement Sara E. Davila, Cesar Davila Hernandez, Martin Flores and Jungseok Ho* Department of Civil Engineering, The University of Texas Rio Grande Valley, Edinburg, TX, USA * Correspondence: Email: [email protected]. Abstract: The intensification of climatic changes, mainly natural geophysical hazards like hurricanes, are of great interest to the South Texas region. Scientists and engineers must protect essential resources from coastal threats, such as storm surge. This study presents the development process and improvements of a hydrodynamic finite element model that covers the South Texas coast, specifically the Lower Laguna Madre, for the aid of local emergency management teams. Four historical tropical cyclone landfalls are evaluated and used as a means of verification of the hydrodynamic model simulation results. The parameters used to improve the accuracy of the model are the tidal harmonic constituents and the surface roughness coefficient, or manning’s n value. A total of four different scenarios that use a variety of tidal constituent combinations and nodal attribute files were developed to identify the best case. Statistical evaluation, such as regression analysis, normalized root mean square error, and scatter index, was used to determine the significance of each hydrodynamic computational storm surge result with observed historical water surface elevations. In an effort to improving all models locally, using seven tidal constituents combinations along with a surface roughness nodal attribute grid that assigns values with respect to bathymetric data improves the accuracy of the storm surge model and should, therefore, be implemented for future hydrodynamic studies in the South Texas region. Keywords: storm surge; tidal constituent; hurricane; South Texas; Lower Laguna Madre 272 1. Introduction The assessment of climate impacts on natural geophysical hazards such as storms and floods are an area of significant interest due to the amount of damage it has caused to coastal areas. Tropical cyclones are the cause of millions of dollars in property damage yearly, so adequate risk assessment of these events is of significant interest to governments, industries, and communities in the area of vulnerability. In the state of Texas, tropical cyclones are responsible for the highest number of deaths of any natural hazard, claiming the lives of 6507 individuals between the years 1851 and 2020. Additionally, they have caused the most property damage of approximately $954.4 billion [1]. Historically, the South Texas region has been very susceptible to these types of natural disasters. In 1967, hurricane Beulah caused 58 deaths as well as $217 million in damages, which is equivalent to $1.59 billion in 2017 currency [2]. Additionally, in 2008 hurricane Dolly caused $1.3 billion in property damages in the United States [3]. Because South Texas is a coastal area with relatively low elevation and narrow stream channels, propagation of storm flood damage is prominent. Further, the Lower Laguna Madre is in this area, and it is essential to preserve this hypersaline lagoon due to the ecological impacts it has on the region [4]. Increasing the reliability of infrastructure systems, whether it be economic, political, and social, depends on the careful determination of surge vulnerability [5]. These natural hazards bring about tides, storm surge, and rain that ultimately are the cause of the damage. Storm surge, which is the abnormal rise in seawater, is one of the most prominent components to flood propagation in South Texas. Flood protection measures should be considered since the developments of this region are not sufficiently designed for extreme surge events [5]. The reason for this is because of how severe these storms are and the insufficient data available to predict the potential damage of these disturbances adequately. Because they do not occur periodically in this region as opposed to rainfall, there is no previous data available about earlier models that have measured hurricane effects, such as storm surge. Developing a coastal storm surge inundation model has the potential to allow emergency responders of the region to improve the resilience of the area. There have been numerous studies that have shown an effort to address natural hazard mitigation through appropriate and accurate storm surge model development. The National Storm Surge Hazard Map developed by the National Hurricane Center (NHC) displays worst-case storm surge flooding scenarios using the National Weather Service (NWS) hydrodynamic storm surge model. This NWS model uses Sea, Lake, and Overland Surges from Hurricanes (SLOSH) to create hypothetical storms using varying conditions to visually map out the inundation across 27 basins in the United States [6]. When a hypothetical Category 4 hurricane like that of Harvey (2017) is implemented into a grid that entails the Texas Coast, an estimated peak surge of 3.84 meters was generated in Calhoun County, Texas, which agrees with actual measurements [7]. The SLOSH model can assist in the validation of the developed South Texas hydrodynamic model by comparing surge heights of the historical and hypothetical hurricane scenarios. A comprehensive storm surge database, SURGEDAT, provides historical storm surge observations for the entire globe [8]. As an example, the SURGEDAT database provides the historical storm surge measurements for hurricanes that have made direct landfalls on the South Texas coast, such as the Dolly (2008) 1.22-meter surge and the Emily (2005) 1.52-meter surge. These measurements are useful to this study because we can use these values to compare and validate the developed model. An Advanced Circulation (ADCIRC) model specific to the Gulf of Mexico region implements hindcast studies, which are dependent on AIMS Geosciences Volume 6, Issue 3, 271–290. 273 specific model input parameters, such as surface roughness coefficients [9]. Additionally, an ADCIRC model was developed for the Houston, Texas area for adequate sea barrier implementations, and values such as the surface roughness were also modified and observed for better accuracy of the model [5]. Although the TxBLEND water circulation model developed by the Texas Water Development Board (TWDB) is not a model designed for storm surge functions, it is a serviceable model to this study since it provides practical information for essential parameters like surface roughness values for the Texas coasts [10]. All these imperative analysis efforts are needed to provide essential data and communicate it to the public effectively. The appropriate selection of parameters will result in the accurate representation of computations from these models and maps. The objective of this paper is to select the best possible input variables that can provide the most accurate representation of extreme water levels during any hurricane event in the South Texas region. Figure 1. Finite element mesh model domain focusing on the South Texas Coast. The red hollow circle indicates the location of the buoy gage station. Coastal modeling is essential to promote conservation and adequate emergency management and planning [9]. Therefore, the primary focus of this project is to assure model accuracy being developed to achieve this data. A hydrodynamic model was adopted for the area of the South Texas AIMS Geosciences Volume 6, Issue 3, 271–290. 274 coast, specifically focusing on areas near the Lower Laguna Madre. Figure 1 entails the Gulf of Mexico in its entirety, with a focus on the Lower Laguna Madre area. All modeling requires a level of engineering judgment, primarily when focusing on the accuracy and model improvements. For this hydrodynamic model, the crucial parameters to focus on for proper calibration and model development is the tidal constituents and surface roughness coefficients. This paper entails the model improvement methodologies and the judgment that was made based on previous literature that has dedicated their time to similar projects. The goal is to improve the current hydrodynamic model developed for the South Texas region by determining the best tidal harmonic constituent combination and the surface roughness of the model domain. These parameters are tested by executing the hydrodynamic model with four historical hurricanes that have made landfall in the South Texas area. The four historical hurricanes include Bret (1999), Dolly (2008), Emily (2005), and Alex (2010). The computational data that is retrieved from the hydrodynamic model execution and then compared to the water surface elevation data provided by the National Oceanic and Atmospheric Administration (NOAA) buoy stations. Statistical analysis, such as linear regression, root mean squared error method, scatter index, and percent increase is used to analyze the accuracy of each computational result. An accurate model would ultimately increase the usefulness to the communities in the nearby locations, for they are using a model that is reliable and accountable for their emergency management planning. 2. Materials and methods The Surface Water Modeling System (SMS) software is used for the pre-processing and post- processing of the finite element mesh development of respective areas [11]. The ADCIRC model is a finite element program that